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Updated: May 29, 2025
Immediately under the thin corium, and closely connected with it, we find a thin muscle tube, as in the worms. On the other hand, the Ascidia has a centralised heart, and in this respect it seems to be more advanced than the Amphioxus. On the ventral side of the gut, some distance behind the gill-crate, there is a spindle-shaped heart.
The whole of the anterior or respiratory section of the gut is converted into a gill-crate, which is pierced trellis-wise by numbers of branchial-holes, as in the ascidia. This is done by the foremost part of the gut-wall joining star-wise with the outer skin, and the formation of clefts at the point of connection, piercing the wall and leading into the gut from without.
Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body. The small particles that the Amphioxus takes in with the water infusoria, diatoms, particles of decomposed plants and animals, etc. pass from the gill-crate into the digestive part of the canal, and are used up as food.
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